Applies to: All Interfaces
Audacity runs off Windows inbuilt audio driver. As the Windows inbuilt audio driver only supports a maximum of two inputs and two outputs, regardless of the input/output count of your Focusrite interface, Audacity will only ever see the first two inputs and outputs of your device.
In short, you can use your Focusrite interface with Audacity, but you will only be able to use the first two inputs and outputs.
Setup Article - Recording in Audacity (Windows)
As OS X’s inbuilt audio driver (Core Audio) is able to support multichannel input/output, Audacity will see all inputs and outputs of any Scarlett / Saffire / Clarett interface.
Note that the way in which Audacity allows you to select which input you are recording off differs slightly from most other recording software;
It is not possible to record an input without Audacity also recording the inputs that come before this. For example, when recording the signal coming into input 6 of your interface, Audacity requires you to record inputs 1,2,3,4,5 and 6.
Setup Article - Recording in Audacity (Mac)